Lab Handout 1: File Systems and System Calls
The first two exercises present problem set-esque questions that could easily appear on a midterm or final exam (or, in your case, your first assessment). In fact, many of the questions asked in Problem 2 were on previous midterms and finals. The third problem is an experiment that'll require you fire up your laptop and run some programs and development tools.
The lab checkoff sheet for all students can be found right here.
Problem 1: Direct, Singly Indirect, and Doubly Indirect Block Numbers
Assume blocks are 512 bytes in size, block numbers are four-byte
ints, and that inodes include space for 6 block numbers. The first three contain direct block numbers, the next two contain singly indirect block numbers, and the final one contains a doubly indirect block number. Note that this is a slightly different scheme than that used in assign2.- What's the maximum file size?
- How large does a file need to be for the relevant inode to require the first singly indirect block number be used?
- How large does a file need to be for the relevant inode to require the first doubly indirect block number be used?
- Draw as detailed an inode as you can if it's to represent a regular file that's 2049 bytes in size.
Problem 2: Short Answer
- Consider the Unix V6 filesystem, which uses 512-byte blocks and 32-byte inodes. Which sector should we read in order to get inode 256? If that sector is an array of inodes, which index should we go to in order to get inode 256? What about inode 345?
- Unlike the Unix V6 filesystem we learned about in class, the ext2/3/4 family of filesystems (commonly used on Linux) use variable-length directory entries:
struct ext3_dir_entry {
uint32_t inode_number;
uint16_t name_length;
uint16_t file_type;
char[] file_name;
};
uint32_t inode_number;
uint16_t name_length;
uint16_t file_type;
char[] file_name;
};
What is the benefit to designing directory entries this way? What are some drawbacks? (Consider what happens when deleting files.)
- Explain what happens when you type
cd.././../.at the shell prompt. Frame your explanation in terms of the file system described in lecture. Assume that the inode number of the current working directory is the only relevant global variable maintained by your shell. - All modern file systems allow symbolic links to exist as shortcuts for longer absolute and relative paths (e.g.
search_solnmight be a symbolic link for/usr/class/cs110/samples/assign1/search_soln, andtests.txtmight be a symbolic link for./mytests/tests.txt. Explain how the absolute pathname resolution process we discussed in lecture would need to change to resolve absolute pathnames to inode numbers when some of the pathname components might be symbolic links. - Recall that the stack frames for system calls are laid out in a different segment of memory than the stack frames for user functions. How are the parameters passed to a system calls? How does the kernel know when to begin executing the system call? And does the system call return and transfer control back to the user program?
Problem 3:
valgrind and orphaned file descriptorsHere's a very short exercise to enhance your understanding of
valgrind and what it can do for you. To get started, type the following in to create a local copy of the repository you'll play with for this problem:poohbear@myth58:~$ git clone /usr/class/cs110/repos/lab1/shared lab1
poohbear@myth58:~$ cd lab1
poohbear@myth58:~$ make
poohbear@myth58:~$ cd lab1
poohbear@myth58:~$ make
Now open the file and trace through the code to keep tabs on what file descriptors are created, properly closed, and orphaned. Then run
valgrind ./open-fds to confirm that there aren't any memory leaks or errors (how could there be?), but then run valgrind --track-fds=yes ./open-fds to get information about the file descriptors that were (intentionally) left open. Without changing the logic of the program, insert as many close statements as necessary so that all file descriptors (including 0, 1, and 2) are properly donated back. In general, you don’t close file descriptors 0, 1, and 2.You can also trace through the problem using an online programming environment that Ryan Eberhardt has built over the last two years. That environment—called C Playground—has been set up for this problem right here.